1. Field of the Invention
This invention relates to a method and apparatus for vacuum-evaporating a photosensitive material and the like and, in particular, to a method for evaporating a multi-element photosensitive material subject to fractional evaporation.
2. Discussion of the Prior Art
In a known vacuum evaporation method for forming a film of a substance on a substrate, the substance is heated in vacuum to evaporate and then deposit on the substrate to form a thin film of the substance. As well known, this method is in practical use in various fields as a film-forming technique. Particularly in xerograhy and its related fields, where selenium is a representative photosensitive material, vacuum evaporation is an essential technique for forming a photosensitive film. The known method has therefore been reviewed with a view to adapting it to the various characteristics required of photosensitive materials.
For forming a photosensitive film of a single element type, use has been made of an apparatus as shown in FIG. 1, in which the inside of a hermetic shell 1 is brought to a vacuum state by a vacuum pump where a quantity of evaporant or material 4 is disposed in a crucible 3. The crucible 3 is made of a known resistant material and heated to evaporate the material 4 by electric power supplied from a power supply 7 including a transformer 5 and a controller 6. A substrate 9 is mounted on a substrate holder 8 to permit deposition of the evaporated material. Although vacuum evaporation by this apparatus has been in use because it is satisfactory for the formation of films of the single element type, recent developments in research and development of multi-element photosensitive materials have rendered this apparatus inadequate.
Multi-element photosensitive materials comprises two or more elements. When these elements are in the form of an alloy, the conventional apparatus of FIG. 1 causes frictional evaporation for each of the elements. Assuming that the photosensitive material has two metallic elements A and B with A being more difficult to evaporate, the distribution of the elements with respect to the thickness of the substrate 9 will be such that B is relatively richer at the deeper portion of the film compared with the original composition of the source material, while A becomes relatively richer toward the surface, thus creating a non-uniform distribution of the elements. In some cases, the composition at the surface of the deposit film may be 40%A - 60%B for an original composition of 10%A - 90%B. The change in composition of the film tends to adversely affect the quality of an electrostatic photocopying process utilizing the film, particularly when the film is subject to abrasion. This is due to the fact that the film comes to have varied compositions depending on the degree of abrasion. Particularly when a photosensitive material, for use in electrostatic photocopying is used, the composition of the film directly affects the quality of the photocopying to such an extent that the quality depends directly on the distribution of the composition. For these reasons, the evaporation apparatus of FIG. 1 has proven to be inadequate. Attention has thus been given to evaporation methods capable of controlling the distribution of the composition while suppressing the temperature-dependent fractional evaporation of different elements.
Conventional fractional-evaporation-suppressed evaporation methods adapted to alloys and the like may be exemplified by flash heating and electron beam heating techniques. However, they are suited only for substrates of relatively small area. Even though applicable in principle to the mass production of substrates of large area, they are difficult to put into practical use because the uniformity of film quality cannot be ensured. Another disadvantage is that these techniques result in apparatus which is complicated, difficult to operate and relatively costly to manufacture.